Marine propulsion

ABSTRACT

A marine propulsion unit comprises a duct having in flow series, a divergent portion, an intermediate and mixing portion in which a centre body is located and a nozzle portion having a convergent-divergent nozzle, the centrebody having in flow series, combustion means and turbine means and being adapted to receive a supply of fuel and compressed air from sources located remotely of the centrebody, the turbine means being arranged to drive an impulse rotor located at the outlet of the nozzle portion and the exhaust from the turbine means being arranged to flow into the mixing portion of the duct. The source of compressed air, which is preferably located in the hull of the vessel can be a single or twin spool gas generator, a gas turbine engine having a tapping from a suitably sized compressor or a gas generator with a power turbine arranged to drive a compressor.

Scott-Scott MARINE PROPULSION [75] Inventor: JohnLanfear Scott-Scott,

Bulkington, England [731' Assignee: Rolls-Royce (1971) Limited,

London, England [22] Filed: Feb. 5, 1973 [21] Appl. N0.: 329,334

[30] Foreign Application Priority Data Feb. 11, 1972 Great Britain .l 6475/72 [52] U.S. Cl 60/221, 115/13, 115/16 [51] Int. Cl B63h ll/08, B63h 11/02 [58] Field of Search 115/11, l2, 13, I4, 15, 115/16; 114/150, 151; 60/221, 222, 226

[56] References Cited UNITED STATES PATENTS 2,447,100 8/1948 Stalker 60/226 3,153,905 10/1964 Campbell..... 3,365,891 1/1968 Williams 115/13 Cushman [111 3,808,804 [451 May 7,1974

Primary Examiner-Trygve M. Blix Assistant Examiner-Charles E. Frankfort Attorney, Agent, or Firm-Cushman, Darby &

57 ABSTRACT A marine propulsion unit comprises a duct having in flow series, a divergent portion, an intermediate and mixing portion in which a centre body is located and a nozzle portion having a convergent-divergent nozzle, the centrebody having in flow series, combustion means and turbine means and being adapted to receive a supply of fuel and compressed air from sources located remotely of the centrebody, the turbine means being arranged to drive an impulse rotor located at the outlet of the nozzle portion and the exhaust from the turbine means being arranged to flow into the mixing portion of the duct.

The source of compressed air, which is preferably located in the hull of the vessel can be a single or twin spool gas generator, a gas turbine engine having a tapping from a suitably sized compressor or a gas generator with a power turbine arranged to drive a compressor.

8 Claims, 8 Drawing Figures PA'TENTEDMAY 7 m4 sum 1 or 3 FIG].

MTENTEBIAY 71914 3 3 SHEET 2 [IF 3 1v MARINE PROPULSION The invention relates to a type of propulsion system for use in high speed'ships, hovercraft, and hydrofoils. The great majority of propulsion units currently used for marine craft use mechanical transmission to some form of screw or duct propulsorsystem. Alternative forms of transmission can be employed but in general are restricted to fairly low shaft horsepowers. For very large powers, typically of the order 100,000 hp. and above, the problems of using conventional transmission systems become severe, particularly if the power has to be transmitted through some form of strut or fairing to an underwater thrust pod. In these cases the use of compressed gas as the power transmission medium can appear attractive, particularly if its energy is used to the maximum advantage. Some-of the ways of using this energy involve the production of two-phase flow within the propulsor, and according to the published literature they fall into two main classes.

The first is the basic hydroduct or underwater ramjet, which is not self-starting and which only shows a reasonable efficiency at high speeds, but also has an upper speed limit because of the high air injection-pressures which becomes necessary. The other class of propulsor, which is an improvement on the basic hydroduct, has some pump or other rotor incorporated in the duct for raising the internal pressure, this pump being driven either from an external power source or internally by means of two-phase turbines for example as shown in UK. Patent specification No. 1,238,995.

These devices still require some external starting device and also have several notable disadvantages. One

is that the gas expansion is approximately isothermal, thus setting an upper limit on the energy transfer for a given inlet pressure condition (the type of compressor used for these units will in general be non-iritercooled and therefore absorb a power nearer to the adiabatic compression case). In addition, most published devices have both external and limited internal diffusion in'the duct inlet, both of which are desirable but, in addition, where machines have one or two stages of pressurising pump further diffusion processes are involved which, because of their associated losses inevitably decrease the overall efficiency of the duct- Where a two-phase turbine drive for the pressurizin'g pump is used, problems of flow separation in the nozzle and blading may occur, producing a non-homogeneous two-phase mixture which may not expand in the propulsion nozzle as required, thus incurring a further efficiency loss.

The object of this invention is to provide a device which overcomes or avoids at least some of the disadvantages of the prior art.

The present invention provides a marine propulsion unit comprising a duct having in flow series, a divergent portion, an intermediate and mixing portion in which a centre body is located and a nozzle portion having a convergent-divergent nozzle, the centre body having in flow series combustion means and turbine means and being adapted to receive a supply of fuel and a supply of compressed air from a source located remotely of the centre body, the turbine means being arranged to drive an impulse rotor located at the outlet of the nozzle portion and the exhaust from the turbine means being arranged to flow into the mixing portion of the duct.

The source of compressed air may takeany suitable form, for example, the source can comprise a gas generator with apower turbine arranged to drive a compressor the outlet from which passes to the centre body or a relatively large gas generator with a facility for bleeding off a large proportion of the compressed air from the compressor section of the gas generator or a gas turbine engine having an extra stage or stages on the compressor and a turbine with a lowpressure drop so that the outlet pressure is substantially the same as that of the two sources already mentioned, the hot ex-- haust from the engine being passed to the centre body.

the compressed air and fuel can pass, the source of compressed air being located in the hull of the vessel.

The propulsion unit or. a number of propulsion'unit s can power any type of marine vessel, e.g. the normal displacement type, or hovercraft o'r hydrofoil but is particularly adapted for propelling relatively large hovercraft at high speeds.

The invention will now be particularly described with reference to the accompanying drawing in which:

FIGS. 1 to 3 show various forms of known two-phase propulsion units, v Y

FIG. 4 shows a diagrammatic layout of one form of propulsion unit, according to the present invention, and

FIGS. 5 to 8 inclusive show diagrammatic layouts 0 various sources of compressed air for the unit shown in FIG. 4.

FIG. 1 shows an arrangement known as a mist jet and compresses a water scoop with injectors which are located in a duct, the duct being arranged to receive a supply of high pressure air, for example from a ducted fan driven by a gas turbine engine or some other convenient power source.

This propulsion unit requires a complex injector system to achieve uniform distribution of water droplets and the presence of the water injector in the high pres- .sure duct imposes a considerable pressure loss on the duct, requiring greater power to drive the fan than would be the case for a single duct. Also, the water scoop has to' extend into the water for a considerable depth to allow for variation in wave height and craft motion thereby imposing a considerable drag on the craft. Information on this type of unit can be found in the following references: A Review of Two-Phase Marine Propulsion R. Meunch and .I. Garret, NSRDC Annapolis A.I.A.A. Paper 72-589, A Wateraugmented Air Jet for the Propulsion of High speed Marine Vehicles R. Meunch and A. Ford, NSRDC Annapolis, A.I.A.A. Paper 69-405, A Preliminary Parametric Study ofa Water-augmented Air Jet for High speed Ship Propulsion R. Meunch and T. Keith, US. Navy Marine Engineering Laboratory, Annapolis, R & D Report 358/66 Feb. 1967.

FIG. 2 shows a hydro-ram jet which has a duct with an air injector, the duct having an inlet diffuser and a downstream nozzle and is the water equivalent of the aerodynamic ram jet.

This propulsion unit is not self-starting and due to difficulties of achieving a fine distribution of gas bubbles, the unit operates with a low thrust coefficient. If the volume of gas injected is increased, the practical limit is reached when bubbles are emitted from both ends of the unit and efficiency falls rapidly.

This type of unit can be improved by fitting a pump in the inlet section of the duct so that the unit is selfstarting and also the pressure at the mixing section is correspondingly higher.

Information on the type of unit shown in FIG. 2 can be found in the following references: A Review of Two- Phase Marine Propulsion R. Meunch and J. Garret, NSRDC Annapolis, A.I.A.A. Paper 72-589, Preliminary investigation of an Underwater Ramjet powered by compressed air, f E. Mottard and C. Shoemaker, NASA TN D-99l, Dec. 1961, A note on Air Blown Water Ramjets, --G. Gadd, NPL Ship Division.

Fig. 3 shows a hydro-turbojet and is the water equivalent of the aircraft gas turbine engine. This type of pro-v pulsion unit comprises a duct in which is located a turbine driven upstream pump and an air injector located in a mixing section of the duct which has an inlet diffuser and a nozzle downstream of theturbine.

The flow field is considered to have some disadvantages notably in the area of the turbine and associated nozzles where it may be deduced that separation of the nominally homogeneous two-phase flow into separate phases can readily occur. On separation, thegas phase will blow through without transmitting'its expansive energy to the liquid flow and the turbine power will be drastically reduced. Information on this type of unit can be found in A Review of Two-phase Marine Propulsion, R. Meunch and J. Garret NSRDC Annapolis, A.I.A.A. Paper 72-589 and UK. Patent No. 1,238,995.

In FIG. 4, a propulsion unit is attached by means of a streamlined hollow duct 12 to a marine vessel (not shown), the propulsion unit being mounted well below the water line and the strut being inclined against the direction of motion indicated by arrow F of the vessel.

Supplies of fuel and compressed air are fed to the combustion means through conduits 28 and 30 respectively in the strut 12 the source of compressed air and store of fuel being located remotely from the unit in the hull of the vessel. The fuel and compressed air are mixed together and burnt and the supply of fuel is controlled in a known manner, eg as in a gas turbine engine.

The exhaust from the turbine means passes into a plenum chamber 32 and thence into two radially extending ducts 34 which are both provided with a plurality of exhaust slots 36 positioned at the downstream side of each duct 34.

In FIG. 5, a gas generator 38 is arranged to drive a power turbine 40 which is coupled to a compressor 42,

the outlet from the compressor being passed to the conduit 30. The power turbine 40 can be coupled to the shaft of the gas generator 38 or it can be a free power turbine.

In FIG. 6, the source comprises a gas turbine engine I 44, having a compressor 46 from which a large proportion of compressed air can be extracted from the downstream end of the compressor and passed to the conduit In FIG. 7, the source is also a gas turbine engine 46, having a compressor 48, having an extra stage or stages 48a, combustion means 50 and a turbine 52 having a relatively low overall pressure drop so that the pressure at the turbine exhaust is comparable to the pressure at the outlet of the sources described in FIGS. 5 and 6.

In FIG. 8, the source is similar to that shown in FIG. 2, except that it is a twin spool gas generator with the spool axes spaced apart laterally, and not co-axial and axially spaced. The spools carry compressors 54,56 and respective turbines, 58,60 an intercooler 55 is located between the compressors 54,56 and a gas generator 62 is provided to drive the turbines 58,60.

In operation, compressed air is supplied to the combustion means, combustion takes place, the turbine is rotated and the impulse rotor is driven causing forward movement of the vessel. Water flows into the duct after subjection to a certain amount of external diffusion in the region directly upstream of the duct inlet, a further amount of diffusion takes place in the portion B and the exhaust from the turbine means flows into the water to form a two-phase mixture. The mixture is accelerated through the nozzle 16 and kinetic energy is added to the fluid by the impulse rotor 18.

The arrangement of the propulsion unit 10 described represents an improvement over the prior art because the only diffusion processes involved are the external effect and a limited amount of internal diffusion in the inlet section. Many of the devices already described in technical literature involve further diffusion processes between pump and stator stages etc., which inevitably cause a larger total loss of energy by the liquid passing through the duct. Considering the fluid which has diffused slightly from the inlet condition and observing from the figure that at this point it is mixed with a suitable proportion of gas supplied bythe power turbine exhaust previously described, the mixture then expands in known manner, that is according to the laws of twophase flow, through an accelerating nozzle which for pressure ratios exceeding a certain calculable value may have a shape as indicated, that is a convergent/divergent passage form. At exit from this nozzle the mixture enters the rotor indicated. When conventional rotors are operated in a two-phase environment it is frequently found that complete flow separation occurs (with consequent loss of energy transfer from the gas to the liquid) on the surface of the blade, that is the non-working surface. In this case, advantage can be taken of this effect and the rotor, which is directly driven by the multi-stage gas turbine, as indicated, operates on the two-phase mixture in the impulse mode, i.e. without any change of static pressure across the rotor. This action is to add kinetic energy to the fluid and therefore accelerate the outgoing liquid to a higher value than that existing at the end of the expansion nozzle. Thus the overall thrust of this propulsion unit is significantly increased above that which would be obtainable in systems not having such a rotor. The design of the rotors suitable for operating in this flow condition, which is termed ventilated flow and has a characteristic similar to super-cavitating flow is known, and is therefore not a subject of the invention.

1 claim:

1. A marine propulsion unit comprising a duct having in flow series, a divergent portion, an intermediate and mixing portion in which a centre body is at least in part located and a nozzle portion having a convergentclivergent nozzle, the centre body being adapted to receive a supply of compressed air and a supply of fuel and having in flow series, combustionmeans and turbine means, the turbine means being arranged to drive 'an impulse rotor located at the downstream end of the nozzle and the exhaust from the turbine means being arranged to flow into the mixing portion of the duct to as claimed in claim 1 in which the propulsion unit is supported by a streamlined strut secured to the vessel.

4. A propulsion unit as claimed in claim 1 in which the compressed air is produced by compressor means located remotely from the duct and centre body.

5. A propulsion unit as claimed in claim 4 in which the compressor means comprises in flow series, a first compressor, combustion means, first compressor driving means and power turbine means, the power turbine means being adapted to drive a second compressor, the outlet of which is arranged to flow t0 the centre body.

6. A propulsion unit as claimed in claim 4 in which the compressed air supply means comprises a gas turbine engine, the compressed air for the centre body being bled from the compressor of the gas turbine engine.

7. A propulsion unit as claimed in claim 4 in which the compressed air supply means comprises a gas turbine engine having at least one extra stage on the compressor and a low pressure drop across the turbine, the exhaust from the turbine being passed to the centre body.

8. A propulsion unit as claimed in claim 4 in which the compressor means comprises a twin-spool gas generator, the spools being parallel to each other and having an intercooler, the two spools being driven bythe exhaust from a gas turbine engine, the exhaust from the twin spool gas generator being passed to the centre body. 

1. A marine propulsion unit comprising a duct having in flow series, a divergent portion, an intermediate and mixing portion in which a centre body is at least in part located and a nozzle portion having a convergent-divergent nozzle, the centre body being adapted to receive a supply of compressed air and a supply of fuel and having in flow series, combustion means and turbine means, the turbine means being arranged to drive an impulse rotor located at the downstream end of the nozzle and the exhaust from the turbine means being arranged to flow into the mixing portion of the duct to provide a two-phase flow.
 2. A marine propulsion unit as Claimed in claim 1 in which gas distributing means are located downstream of the turbine means, the gas distributing means comprising a plenum chamber and at least two radially extending ducts, the ducts having a plurality of outlet apertures for the efflux of gas.
 3. A marine vessel having at least one propulsion unit as claimed in claim 1 in which the propulsion unit is supported by a streamlined strut secured to the vessel.
 4. A propulsion unit as claimed in claim 1 in which the compressed air is produced by compressor means located remotely from the duct and centre body.
 5. A propulsion unit as claimed in claim 4 in which the compressor means comprises in flow series, a first compressor, combustion means, first compressor driving means and power turbine means, the power turbine means being adapted to drive a second compressor, the outlet of which is arranged to flow to the centre body.
 6. A propulsion unit as claimed in claim 4 in which the compressed air supply means comprises a gas turbine engine, the compressed air for the centre body being bled from the compressor of the gas turbine engine.
 7. A propulsion unit as claimed in claim 4 in which the compressed air supply means comprises a gas turbine engine having at least one extra stage on the compressor and a low pressure drop across the turbine, the exhaust from the turbine being passed to the centre body.
 8. A propulsion unit as claimed in claim 4 in which the compressor means comprises a twin spool gas generator, the spools being parallel to each other and having an intercooler, the two spools being driven by the exhaust from a gas turbine engine, the exhaust from the twin spool gas generator being passed to the centre body. 